You can't get HD through an aerial as the bandwidth is too small.
Yes you can, there will be a lot of HD channels available on Freeview eventually. They can't broadcast them at the moment as there isn't any spare signal ranges / broadcast frequencies (or whatever they are called). Once the analogue TV signals are turned off and re-allocated (some of them) to Freeview we'll start seeing HD channels.
Much as I hate to say it, Virgin are pretty hard to beat for broadband. I get pretty much the full advertised 10Mb, which you seldom get with other ISPs - the rates they advertise are best case scenario and seldom achieved in the real world. Virgin is pretty cheap as well if you get a phone line from them as well - £26 for phone and 10Mb I think at the moment. Their PVR recorder is pretty good as well - cheap and its a triple tuner.